1. Field of the Invention
The present invention relates to a long cavity single-mode laser diode having improved longitudinal single-mode stability.
2. Description of the Related Art
A longitudinal single-mode semiconductor laser diode is an essential optical component used in coherent optical communication or in wavelength division multiplexing optical communication.
In general, a filter or grating that works as a wavelength selector is inserted into a cavity to obtain a single-mode in a semiconductor diode having a longitudinal multi-mode, for example, a Fabry-Perrot laser diode.
Various methods of obtaining a single-mode have been reported, and laser diodes can be classified into short cavity laser diodes and long cavity laser diodes. Examples of short cavity laser diodes include distributed feedback laser diodes (DFB-LDs) and distributed Bragg reflector laser diodes (DBR-LDs). Short cavity laser diodes include a grating having a longitudinal refractive index or gain periodicity in a cavity, thereby obtaining a longitudinal single-mode.
Other examples of short cavity laser diodes are distributed reflector laser diodes (DR-LDs), coupled cavity laser diodes (CC-LDs), etc., which have a relatively short length (1 mm or less), thus having high direct modulation speed, and a mode spacing (inversely proportional to the length of the device) is broader than the spectral bandwidth of the grating or filter, and thus a single-mode can be easily obtained.
Long cavity laser diodes can be classified into arrayed waveguide grating-based laser diodes (AWG-based LDs) and concave grating-based laser diodes (CG-based LDs).
AWG-based laser diodes are light sources composed of a semiconductor optical amplifier (SOA) and an AWG which are monolithic-integrated and functioning as a single-mode light source, and CG-based laser diodes include an SOA and a concave grating in monolithic integration.
Long cavity laser diodes include an SOA generating optical gain and an AWG or CG performing wavelength selection in monolithic integration; however, the length of cavities spaced apart from each other is several mm, which is relatively greater than short cavity laser diodes. Thus, the modulation speed is low due to the long round-trip time in the cavities.
Since long cavity laser diodes can be easily manufactured as an array type, they can be used as multi-wavelength laser sources, and can be easily integrated with optical modulators, optical attenuators, and optical detectors, etc., thus having a wide usage range.
FIG. 1 is a schematic view of a conventional AWG-based laser. Referring to FIG. 1, the AWG-based laser is formed of an SOA pre-amp 100, an AWG 110, and a wavelength selection SOA array 120. The SOA pre-amp 100 and the wavelength selection SOA array 120 generate optical gain by applied current, and the AWG 110 selects wavelength.
When a current is injected into the SOA pre-amp 100, optical gain is generated to generate light in a broad wavelength range, and the generated light passes through the AWG 110 and is guided as light having different wavelengths along a output waveguide array by the dispersion characteristics of the AWG 110.
When a current is injected to the wavelength selection SOA array 120, guided beams obtain optical gain to reach a facet on the right side of the wavelength selection SOA array 120 and are reflected back to the AWG 110 and the SOA pre-amp 100. A facet on the left side of the SOA pre-amp 110 and a facet of the right side of the wavelength selection SOA array 120 function as a cavity, and light is outputted through the left facet of the SOA pre-amp 100. (Of course, light is also outputted through the SOA array 120.) When no current is injected to the wavelength selection SOA array 120, guided beams are absorbed and no light is outputted. Accordingly, in FIG. 1, the SOA pre-amp 100 is always switched on, and a beam at a predetermined wavelength can be outputted to the left facet of the SOA pre-amp 100 by injecting current to at least one of the wavelength selection SOA array 120.
FIGS. 2A through 2D illustrate operating characteristics in wavelength domain of the AWG-based laser in FIG. 1.
The wavelength selection transmission characteristics of the AWG 110 has a width of about 0.4 nm to 0.8 nm (50 GHz to 100 GHz) as illustrated in FIG. 2A. The length L of the cavity of the total device is at least 2 mm or greater, the operating wavelength λ is 1550 nm, and the group refractive index ng of the waveguide is 3.7, and the cavity mode spacing (Δλ)(=λ2/(2 ngL)) is about 0.16 nm, which is shorter than the width of the passband of the AWG 110 as illustrated in FIG. 2B.
When the AWG-based laser is resonated only by the transmission characteristics of the AWG 110, a multi-mode can be predicted as illustrated in FIG. 2C. However, a single-mode can also appear in predetermined conditions as illustrated in FIG. 2D and based on experimental data (Reference 1: M. Zinrngibl et al, “Digitally tunabel laser based on the integration of a waveguide grating multiplexer and an optical amplifier,” IEEE Photon. Technol. Lett., vol. 6, no. 4, pp. 516-518, April 1994, Reference 2: M. Zirngibl et al, “Characterization of a multiwavelength waveguide grating router laser,” IEEE Photon. Technol. Lett., vol. 6, no. 9, pp. 1082-1084, September 1994), and this is due to the change in optical gain spectrums by nonlinear characteristic inside the SOA (Reference 3: C. R. Doerr, et al, “Single longitudinal-mode stability via wave mixing in long-cavity semiconductor lasers,” IEEE Photon. Technol. Lett., vol. 7, no. 9, pp. 962-964, September 1995, Reference 4: C. R. Doerr, “Theoretical stability analysis of single-mode operation in uncontrolled mode-selection semiconductor lasers,” IEEE Photon. Technol. Lett., vol. 9, no. 11, pp. 1457-1459, November 1997).
In detail, the nonlinear characteristic of the SOA pre-amp 100 is caused by self-stabilization (Reference 5: R. F. Kazarinov, et al, “Longitudinal mode self-stabilization in semiconductor laser,” J. Appl. Phys. vol. 53, no. 7, pp. 4631-4644, July 1982) and four-wave mixing (Reference 6: A. P. Bogatov, et al, “Anomalous interaction of spectral modes in a semiconductor laser,” IEEE J. Quantum Electron., vol. QE-11, no. 7, pp. 510-515, July 1975).
Self-stabilization refers to a characteristic of the laser oscillating at a predetermined wavelength to maintain the present oscillating state. When the wavelength of the peak value of the transmission characteristic of the AWG 110 in FIG. 2A corresponds to a predetermined wavelength of the cavity mode, the optical gain value at this wavelength becomes significantly greater than the optical gain value of the adjacent cavity mode, thereby increasing the light output of the oscillation mode. Four-wave mixing occurs when the optical gain of the long wavelength side increases at a faster rate than the optical gain of the short wavelength side while beating two or more modes of light when the wavelengths of the AWG transmission characteristic peak value and of the cavity mode do not correspond to each other. About 30 dB or greater side mode suppression ratio (SMSR) can be obtained by the above-mentioned nonlinear characteristic.
FIG. 3 is a schematic view of a conventional CG-based laser. Referring to FIG. 3, the CG-based laser (Reference 7: Oh-kee Kwon et al, “Monolithically integrated grating cavity tunable laser,” IEEE photon. Technol. Lett., vol. 17, no. 9, pp. 1794-1796, 2005) is formed of an SOA 300, a phase control section (PCS) 310, a dispersive element (DE) 320, and a concave grating (CG) 330.
The SOA 300 generates optical gain by injected current (ISOA), and the PCS 310 controls the phase of guiding beams by current injection (IPCS), the DE 320 varies the wavelength by the current application (IDE), and the CG 330 selects the wavelength as described with reference to FIG. 1. The CG-based laser illustrated in FIG. 3 is used as a single-mode light source, and can be used at the same time as a wavelength modulation light source by changing the wavelength of the oscillation beam by current application to the DE 320.
When current is injected to the SOA 300, optical gain is generated and light in a broad wavelength range is generated, the generated light passes through the PCS 310, and beams are spread at point A and incident to the CG 330. Only beams at predetermined wavelengths which are incident on the CG 330 are reflected in the same direction as the incident beams by the diffracting characteristic of the grating. The reflected beams are gathered at point A to be reflected back to the PCS 310 and the SOA 300.
Since the left facet of the SOA 300 and a facet of the CG 330 function as a cavity, light is output to the left cross-section of the SOA 300. The CG-based wavelength variation light source can be formed in various shapes except the DE 320 (Reference 8: Oh-kee Kwon et al, “Monolithically integrated multi-wavelength grating cavity laser,” IEEE photon. Technol. Lett., vol. 17, no. 9, pp. 1788-1790, 2005, Reference 9: J. B. D Soole, et al, “Monolithic InP/InGaAsP/InP grating spectrometoer for the 1.48-1.56 μm wavelength range,” Appl. Phys. Lett. vol. 58, no. 18, pp. 1949-1951, May 1991).
The CG 330 functions as the AWG, and thus an operating characteristic of the CG-based laser is the same as that illustrated in FIGS. 2A through 2D. That is, FIG. 2A corresponds to the diffracting reflection characteristic, FIG. 2B corresponds to the cavity mode characteristics, and FIG. 2D corresponds to the light output characteristics. The CG-based laser can also obtain a side mode suppression ratio of 30 dB or greater due to the nonlinearity of the SOA.
FIGS. 4 and 5 illustrate the light output characteristic of the CG-based laser in FIG. 3 when current is injected.
Referring to FIG. 4, the light output spectrum of the CG-based laser in FIG. 3 (length of the SOA: 800 μm, the length of the cavity: about 4 mm, the diffraction order of the CG: four) measured when the injection current ISOA is 10 mA. The light output is obtained through optical fiber and shows high side mode suppression ratio of about 35 dB or greater.
Referring to FIG. 5, when the injection current (ISOA=300 mA) to the CG-based laser is increased, a multi-mode is generated. The multi-mode is generated more easily when the operating current with respect to an identical structure is increased because high injection current saturates the optical gain of a main mode, and thus nonlinear characteristics cannot contribute to mode selection, which is illustrated in FIG. 2C.
When the CG-based laser is manufactured in a structure having a broad diffracting reflection characteristic of the CG (a structure with a lower diffraction order, long grating period, and short distance between point A and the cross-section of the concave grating, etc.), a multi-mode is generated even with respect to low injection current.
The single-mode characteristic of the long cavity laser diodes relies on the nonlinear characteristic of the SOA, and the nonlinear characteristic is significantly influenced by the structure of the device and the operating conditions, and this indicates that the single-mode stability is not good. Recently, various methods have been suggested to overcome such problems. First, two different AWGs and optical couplers are used to overlap each AWG passband to obtain a good quality single-mode (Reference 10: D. Van Thourhout et al, “Compact digitally tunable laser,” IEEE Photon. Tech. Lett., vol. 15, no. 2, pp. 182-184, February 2003, Reference 11: J. H. den Besten et al, “An integrated 4-channel multiwavelength laser on InP,” IEEE Photon. Techonol. Lett. vol. 15, no. 3, pp. 368-370, March 2003).
Such structure is reported to have high side mode suppression ratio of about 40 dB, and moreover, to increase the number of channels significantly. However, such structure has a large size due to the two AWGs, and it is complicated to tune the passband of the two AWGs to obtain a single-mode. Also, a side peak occurs due to undesired wave-mixing effects.
Next, the single-mode stability can be improved by adding a DiDomenico-Seidel cavity (or Vernier-Michelson cavity) having a narrow filtering characteristic to an AWG-based laser (Reference 12: L. Moller et al, “Multifrequency laser based on integrated Vernier-Michelson cavity for mode stabilization,” IEE Electron Lett. vol. 36, no. 6, pp. 540-542, 2000).
The structure can select only a cavity mode due to the filter characteristic of DiDomenico-Seidel cavity even when the width of the passband of the AWG is relatively wide, and thus an excellent single-mode characteristic is expected. However, in order to realize a DiDomenico-Seidel cavity in an AWG-based laser, elaborate design and fabrication are required, and the size of the device is increased, thereby increasing light loss.